A driving method using an inverter has been employed to control an alternating-current motor using a direct-current power source. Switching of the inverter is controlled by an inverter driving circuit. For example, the alternating-current motor is fed with a voltage switched in accordance with PWM control.
Regarding such PWM control, Japanese Patent Laying-Open No. 2008-22624 (Patent Document 1) discloses a technique for preventing partial discharge from being likely to take place when a motor generator is operated under high-humidity environment, by setting a dead time to be longer than that in a normal state so as to relatively weaken an electric field generated in a gap of coil winding by surface charges.